This invention relates generally to electroluminescent lighting, and more particularly to an improved electroluminescent lamp driver that substantially cancels noise.
Electroluminescent lamps are thin planar light sources which are commonly employed to provide backlighting in the display panels of laptop computers, beepers, watches and a myriad of other commercial products. One product for which electroluminescent lamps are becoming increasingly prevalent is cellular telephones. Generally, many of the cellular telephones which are currently being produced have large display panels. The large display panels allow a user to operate the telephone by selecting options from a displayed menu, or to receive and display data via a wireless Internet service.
FIG. 1 is a cross-sectional view of a typical prior art electroluminescent panel 10. Disposed between a transparent front protective cover 12 and a rear protective cover 22 is a transparent front electrode 14, a phosphor layer 16, a dielectric element 18 and a rear electrode 20. During operation, voltage supply source 24 applies a high AC voltage across front electrode 14 and rear electrode 20, resulting in an electric field therebetween. Due to the electric field, the phosphor atoms in phosphor layer 16 are excited to a higher energy state. When the electric field is removed, the atoms fall back to a lower energy state, emitting photons as visible light in the process.
One of the problems experienced by prior art electroluminescent panels, however, is that they typically produce audible noise. Specifically, front electrode 14 and rear electrode 20 vibrate due to the fact that they are alternately attracted to each other (when an electric field is present) and relaxed (when the electric field is not present). Due to this vibration, air is displaced as the volume between the two electrodes is alternately compressed and then returned to normal. This vibration occurs at twice the frequency of the AC power source, since the compression occurs on both the positive and negative voltage excursions of the AC input. Thus, if the AC power source is operating within the typical range of 60 to 400 Hz, audible noise in the 120 to 800 Hz range is experienced by the user. While such audible noise is unpleasant in most commercial products, it is particularly undesirable if it occurs in a cellular telephone, since a cellular telephone is typically held in very close proximity to the user""s ear and the sound quality of the telephone conversation will suffer.
Therefore, there exists a need for an electroluminescent panel which substantially cancels noise.
In accordance with one embodiment, the present invention relates to a lighting system and method comprising at least two electroluminescent panel regions. Each electroluminescent panel region comprises a pair of electrodes with a phosphorous layer therebetween. The lighting system also comprises a lamp driver comprising a voltage supply source for supplying an AC voltage to the electrodes of each of the electroluminescent panel regions so as to cause the phosphorous layer of each electroluminescent panel regions to emit visible light. The lamp driver is configured to alternately charge the electrodes of a first electroluminescent panel region while simultaneously discharging the electrodes of a second electroluminescent panel region. Preferably, the rate of charging the electrodes of the first electroluminescent panel regions is substantially equal to a rate of discharging the electrodes of the second electroluminescent panel regions.
According to one embodiment of the invention, the electrodes of the electroluminescent panel regions are compressed when the electrodes are charged and are decompressed when the electrodes are discharged. In this case, the lamp driver is configured such that the electrodes of a first electroluminescent panel region are compressed at a rate which is substantially equal to a rate at which the electrodes of a second electroluminescent panel region are decompressed, thereby substantially canceling noise which is generated in prior art electroluminescent panels when air is displaced between the electrodes.
In one embodiment, the lamp driver comprises a plurality of current flow control devices, such as p-channel or n-channel MOSFET transistors, coupled to the electrodes. The lamp driver also preferably comprises a system controller which is coupled to and configured to control the transistors. Specifically, the system controller controls which transistors are turned on and off and when, so as to alternately charge and discharge (thereby alternately compressing and decompressing) the electrodes in contiguous electroluminescent panel regions. The noise cancellation renders the lamp driver and electroluminescent panel regions particularly well-suited for employment in a cellular telephone.